Why Validation set is not used in MATLAB example of Deep learning

I have started the deep learning tutorial with the example in Matlab 2017a of Digits. In that example they didn't use Validation set. What is the reason for using validation set?

Matlab Example

Answer 1

[Duncan Lilley]

As of the R2017b Prerelease, there is now a "ValidationData" parameter to the "trainingOptions" function. This allows the user to pass an image datastore, table, or cell array (depending on your data format) that contains all of your validation data.

The R2017b Prerelease can be downloaded here.